This invention relates to a method of for assembling machine parts and, more particularly, to a method of inserting, pressing or screwing machine parts such as shafts into holes provided in a workpiece.
When a machine or a device is constructed by assembling constituent parts, one of the most basic operations is for inserting a part into another such as placing a shaft into a hole or inserting a shaft through a bearing. FIG. 9 shows an example of prior art apparatus for such an operation, with a robot 85 of the type having a multiply articulated member with arms 1 and 2 adapted to move only within a vertical plane. The first arm 1 is rotatably supported at one end through a bearing 3 by a supporting member 4 and is provided with a servo motor 5 with a decelerator so as to be able to rotate in the direction of .phi.1. At the other end, the first arm 1 rotatably supports one end of the second arm 2 through a bearing 6. The second arm 2 can be rotated in the direction of .phi.2 by means of another servo motor 7 with a decelerator. The other end of the second arm 2 is attached through a bearing 8 to a chuck 9 serving as a mechanical hand rotatable in the direction of .phi.3 by means of still another servo motor 10 with a decelerator. The supporting member 4 itself is rotatably supported by a bearing 16 and is rotatable in the direction of .phi.4 by means of its own servo motor (not shown) with a decelerator. The second arm 2 is itself provided with a bearing 17 within itself so as to be additionally rotatable around the axis of its own extension in the direction of .phi.5 by means of still another servo motor (not shown) with a decelerator. Similarly, the chuck 9 is itself provided with a bearing 18 within itself so as to be additionally rotatable in the direction of .phi.6 by means of still another servo motor (not shown) with a decelerator. In FIG. 9, numeral 11 indicates a workpiece placed on a table 12.
Next, the process of installing a part 13 into the workpiece 11 will be explained with reference to FIGS. 10(a) and 10(b). The workpiece 11 may have been transported by a conveyor belt (not shown) and stopped at the specified position on the table 12. The workpiece 11 is provided with holes 14a, 14b, . . . for accepting various parts 13, each hole having a tapered section 15 at its opening part. The robot 85 is operated such that the chuck 9 picks up the part 13 from a part feeder or the like. The chuck 9 holds the part 13 at an angle according to the angle of insertion into the hole 14a and moves it along the surface of the workpiece 11 as shown in FIG. 10(a). As soon as the position of the hole 14a is sensed, the part 13 is aligned with the hole 14a and the part 13 is directly pushed into it as shown in FIG. 10(b) to a desired depth.
After the insertion of one part is thus completed, another part is similarly inserted into another hole. After all required parts have been inserted to the respectively corresponding holes, the workpiece 11 is carried off from the table 12 by the conveyor belt (not shown).
In the case of a hole which is not vertical, however, the part 13 will be grasped obliquely by the chuck 9 which, as shown in FIG. 11, is not vertically oriented. This generates a torque M on the part 13 between the force P from the chuck 9 and the weight G of the part 13 itself. Such a torque tends to cause a change in the direction of the part 13 with respect to the axis of the chuck 9. Thus, even if the chuck 9 is controlled to be oriented parallel to the hole 14, the part 13 does not necessarily become parallel to it and hence cannot be inserted successfully into it. Moreover, if the part 13 cannot be securely held by the chuck 9, there is an increased possibility of the part 13 falling off the chuck 9.
Since the robot 85 is of the multiply articulated type in the case of the apparatus of FIG. 9, furthermore, the rotary motions of the arms 1 and 2 and the chuck 9 must be combined together to produce a linear motion of the chuck 9 parallel to the hole 14 when the part 13 is inserted into it. This conversion of rotary motions into a linear motion requires a complicated control. In other words, the apparatus requires complicated control circuits and complicated control programming for the servo motors 5, 7, 10, etc.
With such a prior art apparatus, positional adjustments were difficult and maintenance of high accuracy in insertion was equally difficult. As a result, the part 13 would become oriented obliquely to the hole due, for example, to an operational error of the robot 85, making it impossible to be inserted reliably and stably into the intended hole.
Since large torques are generated at the joints (that is, on the bearings and the decelerators) of the robot 85 when the part 13 is inserted into a hole forcibly, furthermore, an apparatus as shown in FIG. 9 may not be adapted for press-fitting. In such a case, the robot 85 is used only for positioning the part 13 at the hole, allowing it to enter it only a little and another apparatus may be required to forcibly push the part 13 deeper into the hole 14. In other words, a single prior art apparatus could not be used for press-fitting.
It is therefore an object of the present invention to provide a method of robotically inserting a machine part such as a shaft perpendicularly into a hole in a workpiece such that the mechanism for moving assembly tools can be much simplified and hence that a part can be reliably and it without developing torque on the part stably inserted into a hole or forcibly pressed into.